1. Field of the Invention
The present invention relates to methods of inspection usable, for example, in the manufacture of devices by lithographic techniques and to methods of manufacturing devices using lithographic techniques.
2. Background Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to monitor the lithographic process, it is necessary to measure parameters of the patterned substrate, for example the overlay error between successive layers formed in or on it. There are various techniques for making measurements of the microscopic structures formed in lithographic processes, including the use of scanning electron microscopes and various specialized tools. One form of specialized inspection tool is a scatterometer in which a beam of radiation is directed onto a target on the surface of the substrate and properties of the scattered or reflected beam are measured. By comparing the properties of the beam before and after it has been reflected or scattered by the substrate, the properties of the substrate can be determined. This can be done, for example, by comparing the reflected beam with data stored in a library of known measurements associated with known substrate properties. Two main types of scatterometer are known. Spectroscopic scatterometers direct a broadband radiation beam onto the substrate and measure the spectrum (intensity as a function of wavelength) of the radiation scattered into a particular narrow angular range. Angularly resolved scatterometers use a monochromatic radiation beam and measure the intensity of the scattered radiation as a function of angle.
The diffraction pattern comprises a plurality of diffraction orders and for the first and higher order diffraction orders, there is a pair of diffraction orders (±1st order, ±2nd order, ±3rd order, etc.). Thus, each pixel for first and higher orders of diffraction has an opposite pixel (making a pair of pixels) having an equal and opposite angle of diffraction. The asymmetry for a given angle of diffraction (or pair of pixels) is obtained by subtracting, from within the same diffraction order, the intensity of a pixel from the intensity of other pixel of pair (measured at the same point in time). The asymmetry of a detected beam can be modeled as a weighted sum of oscillating base functions, and most commonly as a series of harmonics. Conventionally, when detecting the first order diffraction pattern only the first order harmonic is used as higher order harmonics have decreasing significance. Two separate sets of superimposed patterns with opposite biases are used to determine the amplitude of the first order harmonic for each pixel in the detected diffraction pattern and the overlay error. However, ignoring the higher order harmonics can lead to intolerable offsets in the determined overlay error, if the second (or higher) order harmonic is sufficiently large and cannot be neglected.
The amplitude of the base function is known as the K value, and the K value for each pixel is determined resulting in a “K-map”. K values can be determined using a pair of superimposed patterns, and then the K-map reused on a target having only a single set of superimposed patterns. However, if the determined K-value is not accurate (for example due to neglecting higher order harmonics) then the overlay determined will also be inaccurate. Furthermore, the K value may vary across the substrate due to process variation across the substrate, for example due to chemical mechanical polishing, and the K value may be determined at a location distant from the determined overlay error.